Optical storage of digital data is a relatively new technology, concerned with the storage and retrieval of digital information using optical techniques, using a special related (ODD, "optical digital data") medium, such as an ODD disk. By analogy such data is conventionally stored on magnetic media like tapes or disks commonly used with high speed digital computers today.
This disclosure relates to analogous optical media and associated read/write techniques and apparatus which are adapted to record and read-back the digital information using a focused beam of prescribed radiation energy, especially with low power laser equipment.
As workers well know, success in designing and operating such a system depends greatly on the storage medium. Workers in the art have, for some time now, pondered how to develop a satisfactory ODD disk medium, especially one adapted for low power lasers presently available and most practical. This invention teaches a practical ODD disk for this purpose, one adapted for reasonably long stable archival-life (e.g., on the order of 10 or more years) and use for record storage in high speed computer systems of today.
Laser recording media, generally:
Now, requirements for such ODD media are stringent; e.g., involving high bit density and cost-effective information storage at satisfactory write/read rates; especially using low-powered lasers. (Workers recognize the simplicity and power of such laser implementation, where one need only modulate and deflect a laser beam for read/write operations). In related applications (e.g., video disk recording) workers have used lasers and, for certain aspects thereof, have suggested media using a metal film as an "information layer", this layer being softened, melted or vaporized (e.g., thermally-ablated) by a write-laser beam sufficient to form a "void" (a pit, hole, bubble, etc., or other deformation) in the layer as the "bit". Such a film may be coated on the surface of a rotating disk.
Many workers have felt that such laser recording is promising for computer records of various kinds; e.g., the mentioned ODD media. They have predicted that practical systems await media responsive to low-power laser writing and that this in turn depends whether one can find such an information layer (material) able to be melted (or vaporized, etc.) at current practical low-power levels. This may also depend on the radiation efficiency of the associated system and on the thermal regime surrounding the layer (e.g., see Bartoline article cited elsewhere).
Thus, workers have looked for such laser recording materials which exhibit high thermal efficiency (a measure of how much of the heat generated at the recording site remains sufficiently localized to allow "pit" formation), and have long sought-out recording materials with a low melting point and low thermal diffusivity (tellurium, lead, bismuth and indium are examples).
And, since workers prefer to use low-power lasers for such recording (e.g., to enhance the operating life of a laser and minimize its cost and size), it has seemed even more desirable that the metal chosen for such an information (absorber) film have an "outstandingly-low" melting point ("adequately-high sensitivity") so that the desired "void" may be formed with minimal laser power.
Te Absorber films:
Now, some workers have contemplated using Tellurium (Te) absorber films for this, or related, laser-recording for several reasons. Tellurium has an attractively-low melting point (about 450.degree. C.), conducts heat poorly and appears able to provide good sensitivity and S/N ratio; also, it is relatively convenient to deposit as a thin film. Bismuth is also commonly suggested for like reasons. And related alloys (e.g., as Te-Ge, Te-As-Se and Bi-Se) have been suggested as of interest.
Tellurium has a low write-threshold (energy)--when compared with Aluminum, for instance, as well as having a much lower thermal diffusivity--e.g., see U.S. Pat. No. 4,222,071 to Bell, et al; also see "Review of Optical Storage Medial" by Zech, SPIE Vol. 177, Optical Information Storage, 1979, page 56 et sequ.; and also see "Optical Recording Media Review" by Bartoline, page 2 et sequ. of SPIE Vol. 123, "Optical Storage Materials and Methods", 1977).
For instance, this Bartoline article discusses such absorber films ("Ablative Thin Films") along with ten other optical recording means--including "Photo-polymers", that is organic compounds known to undergo photochemical changes which involve a shift in refractive index. The Zech article discusses absorber films arranged and used so that laser-writing forms a "hole" in the absorbing layer, this information being detected according to difference in reflectivity (similarly for Bell patent).
For such known "deformational recording", it appears that the thermal energy delivered by a high intensity radiation beam ("Write-Beam" of a Laser) is such that the "write-site" will soften, melt or ablate, in at least part of the beam cross-section. It is believed that surface tension then causes a "transverse cavitation" (see article by Zech cited above), leading in turn to the formation of a "pit" or hole, usually slightly elliptical. (See "Melting Holes in Metal Films for Real-Time, High Density Data Storage" by Cochran and Ferrier, SPIE Proceedings, August 1977, pages 17-31).
Absorber films:
As one feature hereof, we have discovered a related kind of laser-recording can be done at temperatures well-below the melting point of such an "absorber". And, as a result we have also discovered that materials like gold, having a relatively high melting point and rather good conductivity--characteristics heretofore shunned by workers for the instant purposes--can yield a surprisingly good absorber film; one comparable in sensitivity to a "hitherto-preferred" absorber like Tellurium.
Of course, some have vaguely speculated that "high melting point" materials like Titanium and Gold (also platinum, rhodium, nickel, chromium, manganese and vanadium--e.g., cf. U.S. Pat. No. 4,285,056 to Bell) might be suitable for such "deformation" absorber layers. However, such speculations have paid no heed to the practical problem of "sensitivity" or to how a lower power laser can record on them as mentioned above. Or they have ignored the associated problem of high conductivity (where heat is readily conducted away from the recording site it is wasted, further degrading sensitivity--note a metal like gold has a high conductivity, whereas Ti and Te do not).
More astute observers have acknowledged that such "high-melting-point" metals are quite unlikely as absorber candidates (e.g., as expressed in cited U.S. Pat. No. 4,222,071 where the "low-melting-point" and poor conductivity of Te was heartily endorsed as yielding superior sensitivity and enabling one to record with a low-power laser. Thus, metals such as Au which are the REVERSE; i.e., good heat conductors and high melting point should, in theory, be the worst "absorbers"). Yet, the present invention teaches that just such metals as gold can be used and be superior to known absorbers--e.g., having a sensitivity somewhat equivalent to Te and much superior archival life.
This invention, further, teaches that such absorber films--contrary to everything taught in the art--evidently need form no holes or be otherwise deformed in the course of recording.
Extended archival life:
A major advantage of optical data storage technology is the increased storage capacity it affords; e.g., the order of 100.times. that of magnetic tape. An optical data disk as here contemplated will be assumed as "non-erasable", with information stored permanently thereon for an extended archival life on the order of 10-15 years or more. Such extended life is a goal yet to be reached in the art, though workers would dearly love to do so. The present invention promises media exhibiting this archival life, being especially adapted for optical mass memory and like applications.
By contrast, commonly-suggested absorber metals like Bismuth and Tellurium are known to slowly oxidize and otherwise degrade much too readily in the typical user environment; hence, they are poor candidates for such archival records (e.g., see cited article by Ash et al. 1981; and Zech article; also demonstrated by Example I below). Workers generally realize that Tellurium has particularly poor archival stability--i.e., its read-out quickly degrades with time. This degradation is accelerated in a high humidity environment; and is typically characterized by a rapid increase in overall optical transmission, presumably caused by a general oxidation of the metal, as well as by severe attack on selected bit sites, beginning at "defect sites" in the metal film. And Bismuth is similar.
This archival feature is addressed by this invention which teaches the analogous use of related films of gold, etc.--materials which are quite archival and are stable for extended use as computer information storage media, especially as for such optical data disk records for (as in Table I) computers.
Thus, as a feature of novel application, we contemplate the use of such materials for records exhibiting the desired archival life, i.e., being extremely resistant to oxidation or like environmental degradation, during the contemplated use. Thus, no "loss" of recorded information will occur over extended (storage) life--i.e., reflectivity remains stable enough to "read". No practical storage medium or associated system is yet available which can provide extended archival life; especially where high sensitivity is also required (e.g., the sensitivity of Te or better). The invention teaches just this step forward.
The novel recording media taught herein will be generally assumed as meeting the foregoing criteria; and, where possible, as also meeting one or more of the "Target performance criterial" presented in Table I below. (See also "Optical Properties of Tellurium Films Used for Data Recording" by Ash and Allen, SPIE, Vol. 222, 1980).